Semi-insulating InP single crystals, semiconductor devices having substrates of the crystals and processes for producing the same

ABSTRACT

A semi-insulating InP single crystal, semiconductor device with a substrate of crystal and processes of producing the same are disclosed. Crystal is derived from an undoped InP single crystal intermediate. The intermediate has a concentration of all native Fe, Co and Cr of 0.05 ppmw. The crystal has a resistivity of 1x106 OMEGA xcm or more and a mobility of above 3,000 cm2/Vxs both at 300K. A process of producing the crystal includes a step of heat-treating the intermediate under 6 kg/cm2 of phosphorus vapor pressure. The produced semiconductor device is a MIS device operating in essentially the same high-speed manner as a HEMT.

This application is a divisional of copending application Ser. No.07/661,616, filed on Feb. 28, 1991, now U.S. Pat. No. 5,173,127, theentire contents of which are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a semi-insulating InP single crystalsused in electronic devices, in particular, OEICs (i.e., optolectronicIC), HEMTs (i.e., high electron mobility transistor) and ion implantedFETs (i.e., field effect transistor), to a process of producing the InPsingle crystals, to a semiconductor device and a process of producingthe semiconductor device, and more particularly to a MISFET (i.e., metalinsulator semiconductor FET) and a process of producing the MISFET.

2. Description of the Related Art

For semi-insulating compound semiconductor crystals including Si or S asn-type impurities, a process of adding Fe, Co or Cr as deep acceptors tothe crystal has been industrially used. The principle of semi-insulationis based on a mechanism in which deep acceptors compensate shallowdonors. Thus, it has been believed that the added amount of an elementwhich act as an acceptor must be larger than the content of donors inthe compound semiconductor crystal in order to semi-insulate thecrystal.

However, the amount of Fe, Co or Cr doping the compound semiconductorcrystals for the semi-insulation is preferred to be as small aspossible. This is because Fe, Co and Cr serve as deep acceptors, an ionimplanted electronic devices an substrates made of these Fe, Co or Crdoped compound semiconductor crystals, e.g., an FET, and they reduce theactivation efficiency of implanted ions in. In the case of devicesoperating at high frequencies such as an OEIC or HEMT, Fe, Co and Crdiffuse in the epitaxial layers, trap carriers and deterioratehigh-frequency and high-speed performances.

In addition, Fe, Co and Cr easily segregate so that the concentrationsthereof differ in upper and lower portions of the compound semiconductorcrystals, resulting in the nonuniformity of activation efficiency overthe crystal, and therefore resulting in low yields of compoundsemiconductor crystals doped with Fe, Co or Cr.

Heretofore, Fe doped InP single crystals have been generally used forsemi-insulating InP single crystal substrates for electronic devices.When the concentration of Fe in InP single crystal is less than 0.2ppmw, the resistivity is reduced to below 10⁶ Ω·cm and thesemi-insulation is deteriorated. Thus, Fe had to be doped with more than0.2 ppmw in order to retain the semi-insulation thereof.

Generally, it has been believed that a reduced concentration of all ofFe, Co and Cr in the compound semiconductor single crystal reduces theresistivity of the compound semiconductor single crystal since theconcentration of a native impurity (i.e., residual impurity) providing adonor amounts to a level of reduced concentration of all of the Fe, Coand Cr.

However, the present inventors proposed that the electrically activepoint defect, as well as the compensation by donors and deep acceptorscharacterize the mechanism of semi-insulating the InP single crystal anddiligently studied to discover that controlling the density of the pointdefect by means of heat-treating the InP single crystal caused even amuch lower concentration of the deep acceptors than a prior artconcentration thereof so as to semi-insulate the InP single crystal orcompound semiconductor single crystal.

Thus, the inventors previously provided a process of producing acompound semiconductor having a concentration of 0.2 ppmw or less forall of Fe, Co and Cr and a resistivity of 1×10⁷ Ω·cm or more (seeJapanese patent application SHO. 63-220632). The technique of theJapanese patent application SHO. 63-220632 is a process which includesthe steps of vacuum sealing in a quartz ampoule a compound semiconductorcrystal wafer including a concentration of 0.2 ppmw or less of Fe, Co orCr, placing in the quartz ampoule an element of the compoundsemiconductor crystal wafer or a compound semiconductor crystalincluding the element, and heating the quartz ampoule at 400°-640° C. sothat the pressure in the quartz ampoule is equal to or higher than adissociation pressure of the element of the compound semiconductorcrystal wafer.

Hofmann et al discloses in "Appl. Phys. A 48, pages 315-319 (1989)" thatheat-treating an undoped InP single crystal wafer with a 3.5×10¹⁵ cm⁻³concentration of a carrier at a phosphorous vapor pressure of about 5kg/cm² (i.e., 5 bar) at 900° C. for 80 hr, produced an InP wafer havinga resistivity of 2×10⁷ Ω·cm. This is supposed because the electricallyactive point defect is concerned in the same manner as in the process ofthe Japanese patent application SHO. 63-220632.

The present inventors further studied from the process of the Japanesepatent application SHO. 63-220632 to discover that even heat-treating anundoped InP single crystal including a concentration of 0.05 ppmw orless for all of Fe, Co and Cr failed to semi-insulate the crystal.

In addition, in the process of Hofmann et al, heat-treating an undopedInP single crystal having a 3.5×10¹⁵ cm⁻³ carrier concentrationdeteriorated the mobility from 450 cm² /V·s or more to 300 cm² /V·s orless although the resistivity thereof occasionally was 1×10⁶ Ω·cm ormore. A significantly high carrier concentration of the undoped InPsingle crystal provided a resistivity of 10-1×10⁵ Ω·cm and seldomachieved a resistivity of 1×10⁷ Ω·cm or more. Thus, the presentinventors generally reviewed the results of the studies described aboveand concluded that unless the phosphorous vapor pressure to heattreatment temperature ratio was a limitation, no semi-insulating InPsingle crystal with a sufficient mobility could be obtained.

SUMMARY OF THE INVENTION

The present invention was made on the basis of the above discoveries.

A first aspect of the present invention is to provide an undopedsemi-insulating InP single crystal having a resistivity of 10⁶ Ω·cm ormore at 300K and a mobility of above 3,000 cm² /V., the crystal having aconcentration of 0.05 ppmw or less (i.e., the resolution of an analyzer)of all the native Fe, Co and Cr.

A second aspect of the present invention is to provide a process ofproducing the semi-insulating InP single crystal of the first aspect ofthe present invention, including the steps of placing and vacuum sealingboth an undoped InP single crystal intermediate including aconcentration of 0.05 ppmw or less of all the native Fe, Co and Cr(i.e., a retained impurity) and a predetermined amount of phosphorus ina quartz ampoule; and heating the quartz ampoule so that the absolutephosphorous vapor pressure in the quartz ampoule exceeds 6 kg/cm². TheInP single crystal intermediate is preferably derived from an InPpolycrystal having a carrier concentration of 3×10¹⁵ cm⁻³ or less.

A third aspect of the present invention is to provide a MISFET includinga substrate made of a semi-insulating InP single crystal of the firstaspect of the present invention, an insulating layer formed on the topsurface of the substrate, a gate electrode formed on the insulatinglayer and a source electrode and a drain electrode both formed on thetop surface of the substrate, the source and drain electrodes beingformed on opposite sides of the gate electrode.

A fourth aspect of the present invention is to provide a process ofproducing a MISFET of the third aspect of the present invention. In thestep of producing the insulating layer, the substrate made of the anundoped InP single crystal of the first aspect of the present inventionis placed in a vacuum ampoule, a predetermined amount of phosphorus anda predetermined amount of oxygen gas are added and then the vacuumampoule is heated, preferably to oxidize the surface of the substrateunder a phosphorous vapor pressure to produce the insulating layer.

In accordance with HEMT, a kind of very high-speed electronic device, anInAlAs layer of a high bandgap causes a band bend in an active layercomprising an undoped InGaAs layer to produce a channel of high carrierconcentration in the active layer.

On the other hand, the MISFET of the third aspect of the presentinvention includes a high bandgap layer comprising an insulating layer,e.g., a thermally oxidized layer, of SiN_(x) or SiO₂, of a high bandgapinstead of the InAlAs layer of a HEMT, and an InP single crystalsubstrate having an electron mobility of above 3,000 cm² /V·s instead ofthe active layer of a HEMT. As shown in FIG. 7, an application of apositive voltage to the gate formed on the insulating layer of theMISFET thus causes a band bend in the interface between the insulatinglayer and substrate to produce a channel of high electron density. Sincethe InP single crystal of the substrate is of high purity and has a highmobility, the MISFET has a two-dimensional electron layer of a highelectron mobility to provide essentially the same high-speed electronicdevice as a HEMT. Since the interior of the substrate has asemi-insulation of a resistivity of 1×10⁶ Ω·cm, an isolation of devicesformed in the substrate is facilitated, which is favorable forproduction of an IC. When thermally oxidizing the surface of the InPsingle crystal substrate under the phosphorous vapor pressure producesthe insulating layer, the insulating layer takes in an impurity presenton the top surface of the substrate to reduce the interface trap leveldensity of the substrate, resulting in a higher-speed electronic device.

The InP single crystal of the first aspect of the present invention hashigh resistivity and high mobility although the concentration of all ofFe, Co and Cr is 0.05 ppmw or less. Therefore, it is appropriate for asemi-insulating compound semiconductor substrate for electronic devices.In particular, when it is used for OEIC and HEMT substrates, the Feconcentration thereof is so low that Fe cannot diffuse in an epitaxiallayer. This provides an OEIC and HEMT having good high-frequency andhigh-speed performances. On the other hand, when it is used for an ionimplanted FET, the Fe is low so that the activation ratio of implantedions is increased. In addition, amounts of Fe segregating in upper andlower areas of the InP single crystal are negligible so that theactivation ratio of the implanted ions are uniform over the InP singlecrystal, thus increasing yields of the OEIC and HEMT devices.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph of a relationship between the phosphorous vaporabsolute pressure in heat-treating an undoped InP single crystal and themobility of a final InP single crystal;

FIG. 2 is a graph of the relationship between the resistivity of thefinal InP single crystal and the phosphorous vapor absolute pressure inheat-treating an undoped InP single crystal when an InP single crystalwafer which has been subject to the FIG. 1 heat-treatment is capannealed;

FIG. 3 is a graph of the relationship between the mobility of the finalInP single crystal wafer and the phosphorous vapor pressure inheat-treating an undoped InP single crystal when the InP single crystalwafer which has been subject to the FIG. 1 heat-treatment is capannealed;

FIG. 4 is a graph of the relationship between a phosphorous vaporpressure in heat-treating an InP single crystal produced from raw InPpolycrystals of two different carrier concentrations and the resistivityof the final InP single crystal wafer;

FIG. 5 is a graph of a relationship between the carrier concentration ofa used raw InP polycrystal and the resistivity of an InP single crystalwafer after heat-treatment;

FIG. 6 is a graph of a relationship between the carrier concentration ofa used raw InP polycrystal and the mobility of an InP single crystalwafer after heat-treatment;

FIG. 7 is a schematic diagram of an energy band of a MIS device of thethird aspect of the present inventon; and

FIG. 8 is schematic cross-section of a MISFET of one embodiment of thethird aspect of the present invention.

DESCRIPTION OF THE PREFERRED EMODIMENTS Example 1

An InP single crystal ingot of a concentration of 0.05 ppmw (i.e., theresolution of an analyzer) or less for all of Fe, Co and Cr was grownfrom a raw InP polycrystal of a 1×10¹⁵ cm⁻³ carrier concentration by theLiquid encapsulated Czochralski method.

A 0.5 mm thick as-cut undoped InP single crystal wafer was sliced fromthe InP single crystal ingot. The InP single crystal wafer and a redphosphorus were placed in a quartz ampoule. A gas in the ampoule wasevacuated so that the pressure therein was 1×10⁻⁶ torr. Then, anoxyhydrogen burner melted the open end of the quartz ampoule to seal thequartz ampoule. The amount of the red phosphorous was adjusted so thatthe phosphorous vapor absolute pressure in the quartz ampoule was 15kg/cm² at 900° C., i.e., a heat treatment temperature. Then, the quartzampoule was placed in a closed type horizontal heating furnace, heatedand held at 900° C. for 20 hr and then naturally cooled. The usedhorizontal heating furnace can allow a pressure of up to 100 kg/cm²(gauge pressure). During temperature-increasing and cooling, the heatingfurnace received an argon gas of a pressure corresponding to aphosphorous vapor pressure at an increasing or cooling temperature inorder to maintain a pressure balance inside and outside the quartzampoule and avoid a breakdown in the quartz ampoule.

Example 2

The amount of red phosphorus was adjusted so that the phosphorous vaporabsolute pressure in the quartz ampoule was 7.5 kg/cm² at 900° C. Otherconditions equalled those of Example 1.

A 50 μm square part of the surface of each of InP wafers of Examples 1and 2 was lapped. Then, the resistivity and mobility of the InP waferwere measured at 300K by Van Der Pauw method.

Table 1 shows the result of the measurement:

                  TABLE 1                                                         ______________________________________                                               Phosphorous vapor                                                             absolute pressure                                                             in heat treatment                                                                         Resistivity                                                                              Mobility                                               (kg/cm.sup.2)                                                                             (Ω · cm)                                                                  (cm.sup.2 /V · s)                      ______________________________________                                        Before heat                                                                            --            .sup. 3.9 × 10.sup.-1                                                              4,500                                       treatment                                                                     Example 1                                                                              15            1.5 × 10.sup.7                                                                     4,400                                       Example 2                                                                                7.5         8.9 × 10.sup.6                                                                     4,500                                       Control  5               2 × 10.sup.7                                                                     1,400                                       ______________________________________                                    

Table 1 contains the resistivity and mobility of the control of anundoped InP single crystal which was heat treated at 900° C. under a 5kg/cm² phosphorous vapor absolute pressure for 80 hr.

FIG. 1 is a graph of the measured mobility values.

As seen in FIG. 1, heat-treating the undoped InP single crystal wafer sothat the phosphorous vapor pressure in the heat treatment was above anabsolute 6 kg/cm² produced a semi-insulative InP single crystal of aresistivity of 1×10⁶ or more and an above 3,000 cm² /V·s mobility bothat 300K.

In order to test whether or not the resistivity and mobility of anelectronic device with a substrate of the semi-insulating InP singlecrystal of the high mobility obtained in Examples 1 and 2 weremaintained, a 150 nm thick SiN_(x) layer was deposited on the surface ofeach of the InP wafers, then the InP wafers were cap annealed at 700° C.for 15 hr and the resistivities and mobilities of the InP wafers weremeasured.

FIGS. 2 and 3 represent the result of this measurement. In FIGS. 2 and3, the symbol of a black circle indicates values before the cap annealand the symbol of a white circle indicates values after the cap anneal.FIG. 2 indicates that the resistivity of the InP wafer was constantirrespective of the phosphorous vapor absolute pressure. FIG. 3indicates that the mobility of the InP wafer was slightly reduced butmaintained at 3,200 cm² /V·s or more so as to sufficiently be within apractical range.

Example 3

An InP single crystal ingot with a concentration of 0.05 ppmw or lessfor all of Fe, Co and Cr was grown from a raw InP polycrystal having a1×10¹⁵ cm⁻³ carrier concentration by the Liquid encapsulated Czochralskimethod.

A 0.5 mm thick as-cut undoped InP wafer which had been sliced from theInP ingot was heat treated in essentially the same manner as inExample 1. That is, this InP wafer and red phosphorus were placed in aquartz ampoule. A gas in the ampoule was evacuated so that the pressuretherein was 1×10⁻⁶ torr. Then, an oxyhydrogen burner melted the open endof the quartz ampoule to seal the quartz ampoule. Amounts of the redphosphorus were adjusted so that the phosphorous vapor absolutepressures in the quartz ampoules were 3.0 kg/cm², 7.5 kg/cm² and 15.0kg/cm² at 900° C. Then, the quartz ampoules were placed in a horizontalheating furnace, heated and held at a 900° C. for 20 hr and thennaturally cooled.

A 50 μm square part of the surface of each of the InP wafers was lapped.Then, the resistivity and mobility of the InP wafer were measured at300K by the Van Der Pauw method.

FIG. 4 shows the result of the measurement. In FIG. 4, the symbol of ablack circle indicates the resistivity of the InP wafer of Example 3 andthe symbol of a white triangle indicates the resistivity of an undopedInP single crystal wafer, the control, grown from an InP polycrystalhaving a 5×10¹⁵ cm⁻³ carrier concentration by the liquid encapsulatedCzochralski method and heat-treated in the same manner as in Example 3.

As seen in FIG. 4, heat-treating the InP single crystal wafer producedfrom the raw InP polycrystal of the 5×10¹⁵ cm⁻³ carrier concentrationunder the absolute phosphorous vapor pressure of 7.5 kg/cm² failed toprovide an InP single crystal wafer of a high resistivity and on theother hand, heat-treating the InP single crystal wafer produced from theraw InP polycrystal having the 5×10¹⁴ cm⁻³ carrier concentration underabsolute the phosphorous vapor pressure of 6 kg/cm² failed to provide anInP single crystal wafer of a high resistivity. The resistivities of theboth were 4,000 cm² /V·s or more.

FIG. 5 represents a relationship between carrier concentrations of a rawInP polycrystal and resistivities of a final InP single crystal waferwhen undoped InP single crystal ingots were grown from InP polycrystalsof different carrier concentrations by the liquid encapsulatedCzochralski method and InP single crystal wafers sliced from the InPsingle crystal ingots and the InP single crystal wafers were heattreated under the absolute phosphorous vapor pressure of 7.5 kg/cm² inessentially the same manner as described above in Example 3. FIG. 6represents a relationship between carrier concentrations of the raw InPpolycrystal and the mobilities of the final InP single crystal wafer inthe same case of FIG. 5.

As seen in FIGS. 5 and 6, a carrier concentration of 3×10¹⁵ cm⁻³ or lessof the raw InP polycrystal provides a resistivity of 10⁶ Ω·cm or morefor the final InP single crystal wafer and a mobility of 300 cm² /V·s ormore. As seen in FIG. 6, the mobility of the final InP single crystalwafer increases beyond a carrier concentration of 5×10¹⁵ cm⁻³ becausethe final InP single crystal wafer was not semi-insulating.

Examples 1-3 have described the InP single crystal wafer when the heattreatment temperature was 900° C. However, a semi-insulating InP singlecrystal, semiconductor device with a substrate of crystal and processesof producing them are disclosed. Heat-treating an undoped InP singlecrystal having a concentration of 0.05 ppmw or less for all native Fe,Co and Cr under a phosphorous vapor pressure at other temperaturesprovided a semi-insulating InP single crystal of a resistivity of 1×10⁶Ω·cm or more and a mobility of above 3,000 cm² /V·s both at 300K.

Example 4

Example 4 describes a MISFET with a substrate made of an InP singlecrystal wafer of the first aspect of the present invention and a processfor producing the MISFET.

An undoped n-type InP single crystal ingot with a 2-inch diameter and aconcentration of 0.05 ppmw or less for all of Fe, Co and Cr was grownalong the <100> orientation from a raw InP polycrystal having a carrierconcentration of 1×10¹⁵ cm⁻³ by the Liquid encapsulated Czochralskimethod.

The InP single crystal ingot was sliced in a transverse direction to thepull axis thereof. The sliced InP wafer was washed with an organicsolvent, then etched with bromomethanol and then washed with HF (i.e.,hydrofluoric acid) immediately before an oxidation of the InP wafer. TheInP wafer and red phosphorous were placed in a quartz ampoule. A gas inthe quartz ampoule was evacuated so that the pressure therein was 1×10⁻⁶torr. Then, an oxyhydrogen burner melted the open end of the quartzampoule to seal the quartz ampoule. The amount of the red phosphorouswas adjusted so that the phosphorous vapor absolute pressure in thequartz ampoule was 15 kg/cm² at 900° C., i.e., a heat-treatmenttemperature. Then, the quartz ampoule was placed in the horizontalheating furnace, heated and held at a 900° C. for 20 hr and thennaturally cooled.

A 50 μm square part of the surface of each of InP wafers of Example 4was lapped. Then, the resistivity and mobility of the InP wafer weremeasured at 300K by Van Der Pauw method. The resistivity of the InPsingle crystal wafer was 1.5×10⁷ Ω·cm or more. The mobility thereof was4,400 cm² /V·s.

Then, the semi-insulating InP single crystal wafer was pretreated with aphosphoric acid etchant. Then, the pretreated InP single crystal waferand a predetermined amount of phosphorus were placed in a quartzampoule. A gas in the quartz ampoule was evacuated to vacuum. Then,oxygen gas of a pressure of 1.0 atm was introduced into the quartzampoule. Then, the quartz ampoule was sealed. The predetermined amountof phosphorus was such that the phosphorous vapor pressure was apressure of 0.9 atm in a heating of the quartz ampoule. Then, the quartzampoule was heated at 500° C. for 5 hr, which produced a thermaloxidation film on the surface of the InP single crystal. Then, thequartz ampoule was cooled and then opened. Measuring the thickness ofthe thermal oxidation film provided about 40 nm.

Then, a selective ion implantation through above the thermal oxidationfilm implanted Si ions onto the top surface of the InP single crystalwafer. Then, activation-annealing the InP single crystal wafer produceda pair of n⁺ -contact layers 2a and 2b providing a source domain and adrain domain in an upper area of the InP single crystal wafer. A spacingbetween the n⁺ -contact layers 2a and 2b was 1 μm.

Then, opening part of the thermal oxidation film 3 produced on the topsurface of the InP single crystal wafer 1 caused the n⁺ -contact layers2a and 2b to appear. Then, an Au/Ge layer was vapor deposited atop thethermal oxidation film 3 and the n⁺ -contact layers 2a and 2b. Then, theAu/Ge layer vapor deposited InP single crystal wafer was annealed. Then,patterning the Au/Ge layer formed ohmic electrodes 4a and 4b provided asource electrode and a drain electrode. Then, vapor depositing a 200 nmthick layer of aluminum on the thermal oxidation layer, source electrode4a and drain electrode 4b and then patterning the layer of aluminumproduced a gate electrode 5 to provide a MISFET of FIG. 8.

Checking properties of the resulting MISFET confirmed that the cutofffrequency f_(T) thereof was 30 MHz and it was a very high-speed device.

Example 4 employed the thermal oxidation film as the gate insulationfilm. Alternatively, an SiN_(x) film or SiO₂ film produced bysputtering, plasma-enhanced CVD or the like may be employed.

In addition, since producing the gate insulation film by thermaloxidation gettered an impurity present on the top surface of the InPsingle crystal wafer in the gate insulation film, a repetition of a fewtimes of the cycle of thermal oxidation and etching the resultingthermal oxidation film can clean the top surface of the InP singlecrystal wafer very well. Therefore, the gate insulation film may beproduced after a few repetitions of the cycle.

Example 4 produced the n⁺ -contact layers after a production of the gateinsulation film. However, alternatively the n⁺ -contact layers and thegate insulation film may be sequentially produced.

What is claimed is:
 1. A process of producing a semi-insulating InPsingle crystal, comprising the step of heat treating an undoped InPsingle crystal intermediate having a concentration of all of native Fe,Co and Cr of 0.05 ppmw or less under an absolute phosphorous vaporpressure exceeding 6 kg/cm².
 2. A process as recited in claim 1, whereinthe InP single crystal intermediate is derived from an InP polycrystalhaving a carrier concentration of 3×10¹⁵ cm⁻³ or less.
 3. A process ofproducing a semiconductor device, comprising the steps of:heat treatingan undoped InP single crystal intermediate having a concentration of0.05 ppmw or less for all of native Fe, Co and Cr under an absolutephosphorous vapor pressure exceeding 6 kg/cm² to semi-insulate theintermediate; placing in a quartz ampoule a substrate made of the InPsingle crystal resulting from said heat treating step and apredetermined amount of phosphorus; evacuating a gas from the quartzampoule; introducing oxygen gas into the quartz ampoule; and sealing andthen heating the quartz ampule to produce an oxide film on thesubstrate.
 4. The process as recited in claim 3, wherein the InP singlecrystal intermediate is derived from an InP polycrystal having a carrierconcentration of 3×10¹⁵ cm⁻³ or less.